手写一款符合Promise/A+规范的Promise

手写一款符合Promise/A+规范的Promise

长篇预警！有点长，可以选择性观看。如果对Promise源码不是很清楚，还是推荐从头看，相信你认真从头看到尾，并且去实际操作了，肯定会有收获的。主要是代码部分有点多，不过好多都是重复的，不必担心

Promise的一些用法在此不多赘述，本篇主要带领你手写一个Promise源码，学完你就会发现：Promise没有你想象中的那么难

本篇大概分为以下步骤

• 实现简单的同步Promise
• 增加异步功能
• 增加链式调用then
• 增加catch finally方法
• 增加all race 等方法
• 实现一个promise的延迟对象defer
• 最终测试

实现简单的同步Promise

先大概说一下基本概念：

Promise内部维护着三种状态，即pending,resolved和rejected。初始状态是pending，状态可以有pending--->relolved,或者pending--->rejected.不能从resolve转换为rejected 或者从rejected转换成resolved.

即 只要Promise由pending状态转换为其他状态后，状态就不可变更。

ok.知道了这些后，我们开始手撸代码：

注意观看序号 1 2 3 4 5 ...

function Promise(executor){
    let that = this;
    /** 2 定义初始的一些变量 */
    that.status = 'pending';
    that.value = null;
    that.reason = null;
    
    /** 3 定义初始的成功和失败函数 */
    function resolve(value){
        /** 4 判断状态是不是初始状态pending 
         *    是就转换状态 否则不转换 
         *    确保状态的变化后的不可变性 */
        if(that.status === 'pending'){
            that.status = 'resolved';
            that.value = value;
        }
    }
    function reject(reason){
        /** 5 同上 */
        if(that.status === 'pending'){
            that.status = 'rejected';
            that.reason = reason;
        }
    }
    /**
     * 1 Promise中首先传了一个executor，它是一个函数
     *   executor函数中又传了两个函数，分别是resolve和reject
     *   很显然 resolve是成功回调，reject是失败的回调
     */
    executor(resolve,reject);
}

/** 6 在Promise原型上面定义then方法
 *    then方法上面有两个回调 一个是成功后的方法 另一个是失败后的方法
 *    根据成功或失败的状态去执行相关成功onFilfulled()或者失败onRejected()的回调方法
 */
Promise.prototype.then = function(onFilfulled,onRejected){
    let that = this;
    if(that.status === 'resolved'){
        /** 7 如果状态已经变更为resolved 
         *    说明resolve方法已经被调用
         *    那么此时就执行成功的回调函数onFilfulled
         *    并传入参数 that.value
         * */
        onFilfulled(that.value);
    }
    if(that.status === 'rejected'){
        /** 8 同上
         *    传入参数 that.reason
         */
        onRejected(that.reason);
    }
}

module.exports = Promise;

通过require()引入手撸的Promise

let Promise = require('./myPromise');

let p1 = ()=>{
    return new Promise((resolve,reject)=>{
        resolve('success.1');
    });
}

p1().then((data)=>{
    console.log(data); // 打印 success.1
},(err)=>{
    console.log(err);
});

ok.经调用发现 此代码可以实现部分Promise的功能，但仅仅是同步下才有效果。

那异步呢？ 别急这就来~：

增加异步功能

注意观看序号 1 2 3 4 5 ...

function Promise(executor){
    let that = this;
    that.status = 'pending';
    that.value = null;
    that.reason = null;
    /** 1 因为异步不是立即执行 状态不会变更 成功或失败的回调函数也不会执行
     *    所以先定义好存放成功或失败回调函数的数组 
     *    以便将成功或失败的回调函数先保存起来
     * */
    that.onFilFulledCallbacks = [];
    that.onRejectedCallbacks = [];

    function resolve(value){
        if(that.status === 'pending'){
            that.status = 'resolved';
            that.value = value;
            /** 3 发布
             *    等待状态发生变更
             *    状态变更后 立即执行之前存放在相应数组中所有的成功或失败的回调函数
             *    即 发布
             */
            that.onFilFulledCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    function reject(reason){
        if(that.status === 'pending'){
            that.status = 'rejected';
            that.reason = reason;
            /** 4 同上 */
            that.onRejectedCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    executor(resolve,reject);
}

Promise.prototype.then = function(onFilfulled,onRejected){
    let that = this;
    if(that.status === 'resolved'){
        onFilfulled(that.value);
    }
    if(that.status === 'rejected'){
        onRejected(that.reason);
    }
    /** 2 订阅
     *    因为是异步 状态当时并没有立即变更 所以状态还是pending
     *    此时需要把成功或者失败的回调函数存放到对应的数组中
     *    等待状态变更时 再从数组中拿出来去执行
     *    即 订阅
     *    *存放数组时 为了执行时方便 需要把回调函数的外层包裹一层空函数
     */
    if(that.status === 'pending'){
        that.onFilFulledCallbacks.push(function(){
            onFilfulled(that.value);
        });
    }
    if(that.status === 'pending'){
        that.onRejectedCallbacks.push(function(){
            onRejected(that.reason);
        });
    }
}

module.exports = Promise;

代码测试：

let Promise = require('./myPromise');

let p1 = ()=>{
    return new Promise((resolve,reject)=>{
        setTimeout(function(){
            resolve('success.1');
            // reject('fail.');
        },1500);
    });
}

p1().then((data)=>{
    console.log(data); // success.1
},(err)=>{
    console.log(err);
});

可以看到 1.5s后 执行了resolve() 并打印了success.1，至此，我们实现了异步的Promise.其实这里的实现异步的思想就是发布订阅.

en～ok.高能预警:tiger:.接下来就稍稍复杂了 因为我们要实现链式调用then。 要实现这个功能那我们就要重写then方法，并在then方法中重新返回一个Promise,只有这样，才可以实现多次调用then.而且要新增一个解析返回值是否为promise的函数.

稍微捋下逻辑：

• 如果一个then方法返回一个普通值的话，这个值会传递给下一个then中作为resolve成功的结果
• 如果一个then方法返回一个promise的话，会根据返回的promise是成功还是失败，决定下一个then是成功还是失败

:ok_hand: 上代码：

增加链式调用then

注意观看序号 1 2 3 4 5 ...

function Promise(executor){
    let that = this;
    that.status = 'pending';
    that.value = null;
    that.reason = null;
    that.onFilFulledCallbacks = [];
    that.onRejectedCallbacks = [];

    function resolve(value){
        if(that.status === 'pending'){
            that.status = 'resolved';
            that.value = value;
            that.onFilFulledCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    function reject(reason){
        if(that.status === 'pending'){
            that.status = 'rejected';
            that.reason = reason;
            that.onRejectedCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    executor(resolve,reject);
}

Promise.prototype.then = function(onFilfulled,onRejected){
    let that = this;
    /** 1 让promise2等于一个新的Promise 并将promise2返回 */
    let promise2 = new Promise((resolve,reject)=>{
        if(that.status === 'resolved'){
            /** 2 因为返回了promise2 
             *  并且第3步resolvePromiseRelation函数中传递了promise2
             *  而目前promise2并没有拿到
             *  所以加一个定时器 异步执行 等到promise2拿到后 
             *  再去执行 resolvePromiseRelation()方法 并将promise2传递进去*/
            setTimeout(()=>{
                try{
                    let promise3 = onFilfulled(that.value);
                    /** 3 判断新返回值是什么类型的函数
                     *  并将当前的promise：promise2  新的返回值：promise3 
                     *  和 成功时回调：esolve  失败时回调：reject 作为参数传进去 */
                    resolvePromiseRelation(promise2,promise3,resolve,reject);
                }catch(e){
                    reject(e);
                }
            },0);
        }
        if(that.status === 'rejected'){
            /** 同2 */
            setTimeout(()=>{
                try{
                    let promise3 = onRejected(that.reason);
                    /** 同3*/
                    resolvePromiseRelation(promise2,promise3,resolve,reject);
                }catch(e){
                    reject(e);
                }
            },0);
        }
        if(that.status === 'pending'){
            that.onFilFulledCallbacks.push(function(){
                /** 同2 */
                setTimeout(()=>{
                    try{
                        let promise3 = onFilfulled(that.value);
                        /** 同3*/
                        resolvePromiseRelation(promise2,promise3,resolve,reject);
                    }catch(e){
                        reject(e);
                    }
                },0);
            });
        }
        if(that.status === 'pending'){
            that.onRejectedCallbacks.push(function(){
                 /** 同2 */
                 setTimeout(()=>{
                    try{
                        let promise3 = onRejected(that.reason);
                        /** 同3*/
                        resolvePromiseRelation(promise2,promise3,resolve,reject);
                    }catch(e){
                        reject(e);
                    }
                },0);
            });
        }
    });
    /** 同1 */
    return promise2;
}

function resolvePromiseRelation(promise2,promise3,resolve,reject){
     /** 4 防止自己等待自己 一直循环等待 */
    if(promise2 === promise3){
        return reject(new TypeError('循环引用了!'));
    }
    /**  8 一个标示 表示当前没有被调用过 
     *   确保resolve或者reject后的状态不会再次发生变更
    */ 
    let called;
     /** 5 保证promise3是一个引用类型
      *  判断新返回值promise3的类型 
      *  如果是普通值常量 就直接resolve导出 */
    if(promise3!==null&&(typeof promise3 === 'object'||typeof promise3 === 'function')){
        try{
            /** 6 确保promise3是一个Promise
             *  判断promise3的then方法
             *  如果存在 并且是一个function类型 
             *  就表示promise3是一个Promise */
            if(typeof promise3.then === 'function'){
                /** 9 执行promise3的then方法
                 *  因为promise3也是一个Promise
                 *  需要再次解析promise3的then方法
                 *  直到解析到最后的返回值不是一个Promise类型为止
                 */
                promise3.then(promise3, (promise4)=>{
                    /** 同8 */
                    if(called) return;
                    called = true;
                    /** 10 递归解析新的返回值的类型
                     *  解析到返回值不是一个Promise类型为止
                     */
                    resolvePromiseRelation(promise3,promise4,resolve,reject);
                },(r)=>{
                    /** 同8 */
                    if(called) return;
                    called = true;
                    reject(r);
                });
            }else{
                /** 7 此时promise3是一个普通对象 直接resolve() */
                resolve(promise3);
            }
        }catch(e){
            /** 同8 */
            if(called) return;
            called = true;
            reject(e);
        };
    }else{
        /** 同5 普通值直接resolve()*/
        resolve(promise3);
    }
}

module.exports = Promise;

ok. 至此 我们已经实现了Promsie的异步和链式调用. Promise中比较复杂的部分我们已经搞定了 接下来就是添加一些方法，其实这部分反而没那么复杂了.

catch : catch方法本质上就是一个then方法的变形，只有失败时的回调 没有成功时的回调

finally : finally方法的作用是不管 Promise 对象最后状态如何，都会执行操作.其实说白了就是在then方法的成功和失败的回调函数中都执行该方法就行了.

ok.老规矩 上代码～

增加catch finally方法

function Promise(executor){
    let that = this;
    that.status = 'pending';
    that.value = null;
    that.reason = null;
    that.onFilFulledCallbacks = [];
    that.onRejectedCallbacks = [];

    function resolve(value){
        if(that.status === 'pending'){
            that.status = 'resolved';
            that.value = value;
            that.onFilFulledCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    function reject(reason){
        if(that.status === 'pending'){
            that.status = 'rejected';
            that.reason = reason;
            that.onRejectedCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    executor(resolve,reject);
}

Promise.prototype.then = function(onFilfulled,onRejected){
    /** 2 此处有个坑 如果只写1 不写2的话
     *  会报一个TypeError ：onRejected is not a function
     *  在此处给它一个默认的成功和失败的回调函数就好 */
    onFilfulled = typeof onFilfulled === 'function'?onFilfulled:value=>value;
    onRejected = typeof onRejected ===  'function'?onRejected:err=>{throw err};
    let that = this;
    let promise2 = new Promise((resolve,reject)=>{
        if(that.status === 'resolved'){
            setTimeout(()=>{
                try{
                    let promise3 = onFilfulled(that.value);
                    resolvePromiseRelation(promise2,promise3,resolve,reject);
                }catch(e){
                    reject(e);
                }
            },0);
        }
        if(that.status === 'rejected'){
            setTimeout(()=>{
                try{
                    let promise3 = onRejected(that.reason);
                    resolvePromiseRelation(promise2,promise3,resolve,reject);
                }catch(e){
                    reject(e);
                }
            },0);
        }
        if(that.status === 'pending'){
            that.onFilFulledCallbacks.push(function(){
                setTimeout(()=>{
                    try{
                        let promise3 = onFilfulled(that.value);
                        resolvePromiseRelation(promise2,promise3,resolve,reject);
                    }catch(e){
                        reject(e);
                    }
                },0);
            });
        }
        if(that.status === 'pending'){
            that.onRejectedCallbacks.push(function(){
                 setTimeout(()=>{
                    try{
                        let promise3 = onRejected(that.reason);
                        resolvePromiseRelation(promise2,promise3,resolve,reject);
                    }catch(e){
                        reject(e);
                    }
                },0);
            });
        }
    });
    return promise2;
}

function resolvePromiseRelation(promise2,promise3,resolve,reject){
    if(promise2 === promise3){
        return reject(new TypeError('循环引用了!'));
    }
    let called;
    if(promise3!==null&&(typeof promise3 === 'object'||typeof promise3 === 'function')){
        try{
            if(typeof promise3.then === 'function'){
                promise3.then(promise3, (promise4)=>{
                    if(called) return;
                    called = true;
                    resolvePromiseRelation(promise3,promise4,resolve,reject);
                },(r)=>{
                    if(called) return;
                    called = true;
                    reject(r);
                });
            }else{
                resolve(promise3);
            }
        }catch(e){
            if(called) return;
            called = true;
            reject(e);
        };
    }else{
        resolve(promise3);
    }
}

/** 1 直接返回this的then方法
 *  因为catch只捕获错误 所以resolve直接为null
 *  返回reject就好*/
Promise.prototype.catch = function(errFn){
    return this.then(null,errFn);
}

/** 3 finally实现起来也很简单 
 *  分别在resolve和reject中执行fn就好 
 *  最后再把this返回出去就好
*/
Promise.prototype.finally = function(fn){
    this.then(()=>{
        fn();
    },()=>{
        fn();
    });
    return this;
}

module.exports = Promise;

增加all race 等方法

function Promise(executor){
    let that = this;
    that.status = 'pending';
    that.value = null;
    that.reason = null;
    that.onFilFulledCallbacks = [];
    that.onRejectedCallbacks = [];

    function resolve(value){
        if(that.status === 'pending'){
            that.status = 'resolved';
            that.value = value;
            that.onFilFulledCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    function reject(reason){
        if(that.status === 'pending'){
            that.status = 'rejected';
            that.reason = reason;
            that.onRejectedCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    executor(resolve,reject);
}

Promise.prototype.then = function(onFilfulled,onRejected){
    onFilfulled = typeof onFilfulled === 'function'?onFilfulled:value=>value;
    onRejected = typeof onRejected ===  'function'?onRejected:err=>{throw err};
    let that = this;
    let promise2 = new Promise((resolve,reject)=>{
        if(that.status === 'resolved'){
            setTimeout(()=>{
                try{
                    let promise3 = onFilfulled(that.value);
                    resolvePromiseRelation(promise2,promise3,resolve,reject);
                }catch(e){
                    reject(e);
                }
            },0);
        }
        if(that.status === 'rejected'){
            setTimeout(()=>{
                try{
                    let promise3 = onRejected(that.reason);
                    resolvePromiseRelation(promise2,promise3,resolve,reject);
                }catch(e){
                    reject(e);
                }
            },0);
        }
        if(that.status === 'pending'){
            that.onFilFulledCallbacks.push(function(){
                setTimeout(()=>{
                    try{
                        let promise3 = onFilfulled(that.value);
                        resolvePromiseRelation(promise2,promise3,resolve,reject);
                    }catch(e){
                        reject(e);
                    }
                },0);
            });
        }
        if(that.status === 'pending'){
            that.onRejectedCallbacks.push(function(){
                 setTimeout(()=>{
                    try{
                        let promise3 = onRejected(that.reason);
                        resolvePromiseRelation(promise2,promise3,resolve,reject);
                    }catch(e){
                        reject(e);
                    }
                },0);
            });
        }
    });
    return promise2;
}

function resolvePromiseRelation(promise2,promise3,resolve,reject){
    if(promise2 === promise3){
        return reject(new TypeError('循环引用了!'));
    }
    let called;
    if(promise3!==null&&(typeof promise3 === 'object'||typeof promise3 === 'function')){
        try{
            if(typeof promise3.then === 'function'){
                promise3.then(promise3, (promise4)=>{
                    if(called) return;
                    called = true;
                    resolvePromiseRelation(promise3,promise4,resolve,reject);
                },(r)=>{
                    if(called) return;
                    called = true;
                    reject(r);
                });
            }else{
                resolve(promise3);
            }
        }catch(e){
            if(called) return;
            called = true;
            reject(e);
        };
    }else{
        resolve(promise3);
    }
}

Promise.prototype.catch = function(errFn){
    return this.then(null,errFn);
}
Promise.prototype.finally = function(fn){
    this.then(()=>{
        fn();
    },()=>{
        fn();
    });
    return this;
}

/** 1 直接在构造函数上增加all方法
 *  它返回的也是一个Promise
 *  等待参数数组中所有的promise都执行完毕后
 *  再返回结果
 */
Promise.all = function(values){
    return new Promise((resolve,reject)=>{
        /** 2 定义一个存放最终结果的数组result和一个index */ 
        let results = [];
        let index = 0;
        /** 3 定义一个方法addToArr()  
         *  让index每次执行增加results数组元素的函数的时候都+1
         *  当index === values的长度的时候 说明此时所有promsie都执行完毕并放到的数组中
         *  然后直接resolve(results)就行了
        */
        function addToArr(key,value){
            index++;
            results[key] = value;
            /** 6 当满足条件时 说明所有的promise都执行完毕 直接resolve(results) */
            if(index === values.length){
                resolve(results);
            }
        }
        /** 4 循环values中的每一项promsie */
        for(let i = 0; i < values.length; i++){
            let current = values[i];
            /** 5 判断每一项promise的返回值是不是一个Promsie
             *  是的话就执行该Promise的then方法 拿到返回值 并放到数组results中
             *  是一个普通值的话就直接将该值放到数组results中
             */
            if(current && current.then && typeof current.then === 'function'){
                current.then((value)=>{
                    /** 同5 把返回值放到数组results中*/
                    addToArr(i,value);
                },reject);
            }else{
                /** 同5 把返回值放到数组results中*/
                addToArr(i,current);
            }
        }
    });
}

/** race方法相比较于all方法简单很多
 *  因为race中的promsie成功resolve一个 
 *  整个race就resolve */
Promise.race = function(values){
    return new Promise((resolve,reject)=>{
        /** 同4 */
        for(let i = 0; i < values.length; i++){
            let current = values[i];
            /** 同5 */
            if(current&&current.then&&typeof current.then === 'function'){
                /** 7 直接执行then就好 */
                current.then(resolve,reject);
            }else{
                /** 8 普通值直接resolve */
                resolve(current);
            }
        }
    });
}
module.exports = Promise;

实现一个promise的延迟对象defer

此步是为了测试我们手写的Promsie符不符合Promsie/A+规范，如果没有defer的话，我们在测试过程中就会报一个TypeError: adapter.deferred is not a function.

其实写完defer后，我们就可以去进行测试我们手写的Promsie符不符合Promsie/A+规范了。

即：本篇手写一款符合Promise/A+规范的Promise的最终本为：

function Promise(executor){
    let that = this;
    that.status = 'pending';
    that.value = null;
    that.reason = null;
    that.onFilFulledCallbacks = [];
    that.onRejectedCallbacks = [];

    function resolve(value){
        if(that.status === 'pending'){
            that.status = 'resolved';
            that.value = value;
            that.onFilFulledCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    function reject(reason){
        if(that.status === 'pending'){
            that.status = 'rejected';
            that.reason = reason;
            that.onRejectedCallbacks.forEach((fn)=>{
                fn();
            });
        }
    }
    executor(resolve,reject);
}

Promise.prototype.then = function(onFilfulled,onRejected){
    onFilfulled = typeof onFilfulled === 'function'?onFilfulled:value=>value;
    onRejected = typeof onRejected === 'function'?onRejected:err=>{throw err};
    let that = this;
    let promise2 = new Promise((resolve,reject)=>{
        if(that.status === 'resolved'){
            setTimeout(()=>{
                try{
                    let promise3 = onFilfulled(that.value);
                    resolvePromiseRelation(promise2,promise3,resolve,reject);
                }catch(e){
                    reject(e);
                }
            },0);
        }
        if(that.status === 'rejected'){
            setTimeout(()=>{
                try{
                    let promise3 = onRejected(that.reason);
                    resolvePromiseRelation(promise2,promise3,resolve,reject);
                }catch(e){
                    reject(e);
                }
            },0);
        }
        if(that.status === 'pending'){
            that.onFilFulledCallbacks.push(function(){
                setTimeout(()=>{
                    try{
                        let promise3 = onFilfulled(that.value);
                        resolvePromiseRelation(promise2,promise3,resolve,reject);
                    }catch(e){
                        reject(e);
                    }
                },0);
            });
            that.onRejectedCallbacks.push(function(){
                setTimeout(()=>{
                   try{
                       let promise3 = onRejected(that.reason);
                       resolvePromiseRelation(promise2,promise3,resolve,reject);
                   }catch(e){
                       reject(e);
                   }
               },0);
           });
        }
    });
    return promise2;
}

function resolvePromiseRelation(promise2,promise3,resolve,reject){
    if(promise2 == promise3){
        return reject(new TypeError('循环引用了!'));
    }
    let called;
    if(promise3!==null&&(typeof promise3 === 'object' || typeof promise3 === 'function')){
        try{
            if(typeof promise3.then === 'function'){
                promise3.then.call(promise3, (promise4)=>{
                    if(called) return;
                    called = true;
                    resolvePromiseRelation(promise3,promise4,resolve,reject);
                },(r)=>{
                    if(called) return;
                    called = true;
                    reject(r);
                });
            }else{
                resolve(promise3);
            }
        }catch(e){
            if(called) return;
            called = true;
            reject(e);
        };
    }else{
        resolve(promise3);
    }
}

Promise.prototype.catch = function(errFn){
    return this.then(null,errFn);
}
Promise.prototype.finally = function(fn){
    this.then(()=>{
        fn();
    },()=>{
        fn();
    });
    return this;
}

Promise.all = function(values){
    return new Promise((resolve,reject)=>{
        let results = [];
        let index = 0;
        function addToArr(key,value){
            index++;
            results[key] = value;
            if(index === values.length){
                resolve(results);
            }
        }
        for(let i = 0; i < values.length; i++){
            let current = values[i];
            if(current && current.then && typeof current.then === 'function'){
                current.then((value)=>{
                    addToArr(i,value);
                },reject);
            }else{
                addToArr(i,current);
            }
        }
    });
}

Promise.race = function(values){
    return new Promise((resolve,reject)=>{
        for(let i = 0; i < values.length; i++){
            let current = values[i];
            if(current&&current.then&&typeof current.then === 'function'){
                current.then(resolve,reject);
            }else{
                resolve(current);
            }
        }
    });
}
// 实现一个promise的延迟对象 defer
Promise.defer = Promise.deferred = function(){
    let dfd = {};
    dfd.promise = new Promise((resolve, reject)=>{
        dfd.resolve = resolve;
        dfd.reject = reject;
    });
    return dfd;
}
module.exports = Promise;

最终测试

• 测试当前代码是否符合Promise/A+规范
• 全局安装 npm i -g promises-aplus-tests
• 文件所在目录运行以下命令 (例如你的文件名为:MyPrommise.js)
• promise-aplus-tests MyPrommise.js
• 等待
• ok.

  源码在github上，写文章不易，欢迎star或fork，thx～

  github